There have for many years been used large numbers of electrical plug-and-socket combinations including banana plugs. And, presumably for the same many years, the plugs have been abused. They have been accidentally stepped on, pinched, etc., in such manners as to make their springs become at least somewhat flattened. This has caused the effectiveness and usability of the abused plugs to be reduced or eliminated.
Attempts to make banana plugs resistant to such abuse have up until now been unsuccessful, or only partly successful. There has long been a need for a successful--but inexpensive--solution to the abuse problem. The need has been a significant one, especially because when a banana plug goes out of service, so also do the associated wire (lead) and circuitry.